1. Field of the Invention
The present invention relates to an image-recording material capable of obtaining a printing out image having good visibility after image exposure, a lithographic printing plate precursor, and a lithographic printing method of printing by using the lithographic printing plate precursor.
2. Background Art
A lithographic printing plate generally comprises a lipophilic image area that receives ink during printing process and a hydrophilic non-image area that receives a fountain solution. Lithographic printing is a printing method of making difference in ink-adhering property on the surface of a lithographic printing plate with the lipophilic image area of the lithographic printing plate as the ink-receptive area and the hydrophilic non-image area as the fountain solution-receptive area (an ink-repellent area) by making use of the natures of water and oily ink of repelling to each other, adhering ink only on the image area, and transferring the ink to the material to be printed, e.g., paper.
For manufacturing this lithographic printing plate, a lithographic printing plate precursor (a PS plate) comprising a hydrophilic support having provided thereon a lipophilic photosensitive resin layer (an image-recording layer) has so far been widely used. A lithographic printing plate is generally obtained by a plate making method of exposing a lithographic printing plate precursor through an original image of a lith film and the like, and then, for leaving the image-recording layer of the image area, dissolving and removing the image-recording layer of the non-image area with an alkali developing solution or an organic solvent to whereby bare a hydrophilic support surface.
In a conventional plate making process of a lithographic printing plate precursor, a process of dissolving and removing a non-image area with a developing solution and the like corresponding to the image-record layer after exposure is necessary, but the exclusion or simplification of such an additional wet process is one of the objects in the industry. Since the discard of waste solutions discharged with wet processes is a particularly great interest in the industry at large in recent years from the consideration of the global environmental protection, the solution of the above problem is increasingly desired.
For coping with this problem, as a non-processing (non-development) type plate making process that does not necessitate a wet process, a lithographic printing plate precursor having an image-recording layer whose affinity with a fountain solution or ink changes on the surface by exposure and capable of printing without being accompanied by the removal of an image-recording layer is proposed.
As a simple plate-making method, a method that is called on-press development is proposed, which is a method of using an image-recording layer capable of removal of a non-image area of a lithographic printing plate precursor in an ordinary printing process, and removing a non-image area after exposure on a printing machine to whereby obtain a lithographic printing plate.
As the specific examples of on-press development, e.g., a method of using a lithographic printing plate precursor having an image-recording layer soluble or dispersible in, e.g., a fountain solution, an ink solvent or an emulsified product of a fountain solution and ink, a method of mechanically removing an image-recording layer by the contact with the rollers and the blanket of a printing machine, and a method of mechanically removing an image-recording layer by the contact with the rollers and the blanket after weakening the cohesive strength of an image-recording layer or the adhesive strength of an image-recording layer and a support by the permeation of a fountain solution, an ink solvent and the like are exemplified.
In the present invention, unless otherwise indicted, “development process” means a process of removing an unexposed portion with an infrared laser of a lithographic printing plate precursor by being brought into contact with a liquid (generally an alkali developing solution) to thereby bare the hydrophilic support surface by using an apparatus other than a printing machine (generally an automatic processor), and “on-press development” means a method and a process of removing an unexposed portion with an infrared laser of a lithographic printing plate precursor by being brought into contact with a liquid (generally printing ink and/or a fountain solution) to thereby bare the hydrophilic support surface by using a printing machine.
However, when a conventional image-recording layer of an image-recording system utilizing ultraviolet rays and visible rays is used, since the image-recording layer is not fixed after exposure, it is necessary to take a method requiring much labor, such that the exposed lithographic printing plate precursor must be stored under a completely light-shielding condition or a constant temperature condition until it is mounted on a printing machine.
On the other hand, in recent years, digitized techniques of electronically processing, accumulating and outputting image data by using a computer have prevailed, and various image output systems corresponding to these digitized techniques have been put to practical use. Under such circumstances, a computer-to-plate technique of directly making a printing plate without using a lith film is attracting public attention, which technique comprises scanning exposing a lithographic priming plate precursor with high convergent radiant rays such as laser beams carrying digitized image data. With such a tendency, it is an important technical subject to obtain a lithographic printing plate precursor well adapted to such a technique.
Accordingly, in recent years, the simplification of plate-making operation, and the realization of dry system and non-processing system have been more and more strongly required from both aspects of the above-described global environmental protection and the adaptation for digitization.
Since high output lasers such as semiconductor lasers and YAG lasers radiating infrared rays of the wavelength of from 760 to 1,200 nm are inexpensively available nowadays, methods of using these high output lasers as image recording means are now promising as the manufacturing method of a lithographic printing plate by scanning exposure that is easy to be included in digitized techniques.
In conventional plate-making methods, image recording is performed by imagewise exposing a photosensitive lithographic printing plate precursor by low to middle intensity of illumination to cause imagewise change of physical properties by photochemical reaction in the image-recording layer. While in the above method of using high output lasers, an exposure area is irradiated with a great quantity of light energy in an extremely short period of time, and the light energy is efficiently converted to beat energy, the heat energy causes thermal changes such as chemical changes, phase changes and morphological or structural changes in the image-recording layer, and these changes are utilized in image-recording. Accordingly, image data are inputted by light energy, e.g. laser beams, but image recording is performed in the state including the reaction by heat energy in addition to light energy. A recording system making use of heat generation by such high power density exposure is generally called heat mode recording, and converting light energy to heat energy is called light/heat conversion. Such an image-recording layer is also called a image-recording layer in the invention.
Great advantages of a plate-making method using heat mode recording are that image-recording layers are insensitive to the lights of ordinary levels of illuminance such as room illumination, and that the fixation of images recorded by high illuminance exposure is not necessary. That is, lithographic printing plate precursors for use in heat mode recording are free of sensitization by room illumination before exposure and fixation of images is not essential after exposure. Therefore, for example, when a plate-making process is performed by on-press development with an image-recording layer that is insolubilized or solubilized by exposure with high output laser beams, and the exposed image-recording layer is made an imagewise lithographic printing plate, a printing system that an image is not influenced even if exposed to room light after exposure becomes possible. Therefore, it is expected that a lithographic printing plate precursor preferably used for on-press development can be obtained if heat mode recording is used.
As one example, a lithographic printing plate precursor comprising a hydrophilic support having provided thereon an image-forming layer containing hydrophobic thermoplastic polymer particles dispersed in a hydrophilic binder is disclosed in Japanese Patent 2938397. Japanese Patent 2938397 discloses that it is possible to perform on-press development with a fountain solution and/or ink by subjecting the lithographic printing plate precursor to exposure with an infrared laser to coalesce the hydrophobic thermoplastic polymer particles by heat to thereby form an image, and then mounting the lithographic printing plate precursor on the cylinder of a printing machine.
However, it was found that a method of forming an image by coalescence of fine particles by mere heat fusion as above certainly shows a good on-press developing property, but image strength is weak and press life is insufficient.
Therefore, the improvement of press life by making use of polymerization reaction is propose& For example, a lithographic printing plate precursor comprising a hydrophilic support having thereon an image-recording layer (a heat-sensitive layer) containing microcapsules containing a polymerizable compound is disclosed JP-A-2001-277740 (The term “JP-A” as used herein refers to an “unexamined published Japanese patent application”). Further, JP-A-2002-287334 discloses a lithographic printing plate precursor comprising a support having provided thereon an image-recording layer (a photosensitive layer) containing an infrared absorber, a radical polymerization initiator and a polymerizable compound.
In general, as the preprocess of mounting a printing plate on a printing machine, the inspection and discrimination of images on a printing plate, i.e., works for ascertaining whether the images fitting for the purpose are recorded on the printing plate or not, and ascertaining for what a color of ink the plate is, are operated. In ordinary lithographic printing plate precursors including a development process, an image can be easily ascertained after plate-making (after development process), or before printing (before a printing plate is mounted on the printing machine) generally by coloring an image-recording layer in advance.
However, in a lithographic printing plate precursor of an on-press development type or a non-processing (non-development) type not accompanied by development process before printing, the discrimination of a plate cannot be done, since there is no image on the printing plate, which sometimes leads to the error in operation. In particular in multicolor printing, it is important for printing work to be capable of distinguishing whether register marks for register are clearly written so as to be distinguished or not. The invention aims at solving this problem.